Selection in Natural Populations

By Jeffry B. Mitton | Go to book overview
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2
Classes of Abundant Genetic Variation

On an absolute temperature scale the earth is extremely cold, and the chemical transformations which comprise metabolism simply cannot occur at life- supporting rates in the absence of biological catalysts.

P. W. Hochachka and G. N. Somero, Strategies of Biochemical Adaptation ( 1973)

For more than three decades, population geneticists have been using molecular techniques to estimate the levels of genetic variation in populations. Fisheries biologists ( Frydenberg et al. 1965; Sick 1961, 1965a, b) and human geneticists ( Allison 1955, 1964; Boyer 1961; Fildes and Parr 1963; Hopkinson, Spencer, and Harris 1964; Pauling et al. 1949; Robson and Harris 1965; Spencer, Hopkinson and Harris 1964) used electrophoresis to describe population structure before Johnson et al. ( 1966a, b), Harris ( 1966), and Lewontin and Hubby ( 1966) first used electrophoretic surveys of proteins to estimate levels of genetic variation. Electrophoretic analyses of more than 3,000 species showed the percentage range of polymorphic loci to be from zero to 100%, with a mean between 33 and 50% ( Brown 1979, Hamrick and Godt 1990; Hamrick, Mitton and Linhart 1979; Johnson 1976; Nevo 1978; Nevo, Beiles, and Ben-Shlomo 1984; Powell 1975; Selander 1976; see chapter 9). Two concerns, however, have haunted the use of electrophoresis to estimate genetic variability--accuracy and generality.


Accuracy

Because electrophoresis in starch and acrylamide separates proteins on the basis of size and charge ( Poulik 1957) and, to a smaller degree, conformation ( Johnson 1977, 1979). amino acid replacements that do not alter the molecule's size, charge, and conformation will not be detected by electrophoresis ( Coyne 1982; Johnson 1977). The assumption that amino acid replacements would be random with respect to the charge of amino acids led to the conclusion that electrophoretic surveys detect only 30% of the actual number of alleles ( Nei 1975).

More genetic variation can be detected with electrophoresis if proteins are examined in a sequence of different buffer systems. For example, sequential electrophoresis

-14-

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